Demand-based charging of a heat pipe

ABSTRACT

A heat pipe includes one or more reservoirs of liquid that are closed at lower temperatures and open at higher temperatures. The opening of the reservoirs at higher temperatures caused by higher power levels dynamically increases the amount of liquid in the heat pipe, which increases performance of the heat pipe at higher power levels. As the heat pipe cools, the liquid condenses and flows back into the reservoirs. As the heat pipe continues to cool, the reservoirs close. The result is a heat pipe that is more efficient at lower power levels and still maintains high efficiency at higher power levels due to the demand-based charging of the liquid based on temperature.

BACKGROUND 1. Technical Field

This disclosure generally relates to heat pipes, and more specificallyrelates to a heat pipe that includes one or more reservoirs that providedemand-based charging.

2. Background Art

A heat pipe is used to transfer heat between a hot interface and a coldinterface. The heat pipe includes a liquid in contact with a thermallyconductive solid surface at the hot interface. When the hot interfaceheats up, the liquid turns into a vapor by absorbing heat from the hotinterface. The vapor then travels along the heat pipe to the coldinterface and condenses back into liquid, which releases the latentheat. The liquid then returns to the hot interface, and the cyclerepeats. Heat pipes are highly effective thermal conductors, with aneffective thermal conductivity orders of magnitude larger than for otherheat transfer methods, such as a solid metal like copper.

Heat pipes are charged with a liquid. The amount of liquid in the heatpipe determines the performance of the heat pipe. As the rate of heatenergy absorbed by the heat pipe increases, there is a possibility allof the liquid will turn to vapor. At this point, the temperature of thevapor within the pipe will begin to rapidly increase. As a result, thethermal resistance of the heat pipe increases exponentially.Consequently, most known heat pipes are usually overcharged or saturatedwith the liquid to avoid the increase of thermal resistance caused byturning all of the liquid into vapor. However, at lower heat energyrates, the performance of heat pipes that are overcharged or saturatedis less than heat pipes that are charged with less liquid.

Heat pipes are commonly used in heat sinks for modern electronics, suchas processors. To assure the heat sinks work properly when the processoris functioning at high power, the heat pipes in heat sinks are typicallyovercharged or saturated with liquid. This same heat pipe will work lessefficiently at a lower power, meaning the temperature of the processorwill be higher than if a heat pipe that were less charged with liquidwere used. Thus, the designer of a heat sink that uses a heat pipe mustmake a tradeoff between performance of the heat sink at lower powers andperformance of the heat sink at higher powers. Because excessively hightemperatures can cause a catastrophic failure in a processor, thedecision is usually made to overcharge or saturate the heat pipes in aprocessor heat sink so they can handle maximum processor power.

SUMMARY

A heat pipe includes one or more reservoirs of liquid that are closed atlower temperatures and open at higher temperatures. The opening of thereservoirs at higher temperatures caused by higher power levelsdynamically increases the amount of liquid in the heat pipe, whichincreases performance of the heat pipe at higher power levels. As theheat pipe cools, the liquid condenses and flows back into thereservoirs. As the heat pipe continues to cool, the reservoirs close.The result is a heat pipe that is more efficient at lower power levelsand still maintains high efficiency at higher power levels due to thedemand-based charging of the liquid based on temperature.

The foregoing and other features and advantages will be apparent fromthe following more particular description, as illustrated in theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING(S)

The disclosure will be described in conjunction with the appendeddrawings, where like designations denote like elements, and:

FIG. 1 is a block diagram of a heat sink that includes a heat pipe;

FIG. 2 is a graph showing thermal resistance as a function of power forthe heat sink in FIG. 1 based on different levels of liquid in the heatpipe;

FIG. 3 is a block diagram of a heat sink that includes a heat pipe thathas reservoirs of liquid that are closed;

FIG. 4 is a block diagram of the heat sink in FIG. 3 with reservoirs ofliquid that are open when all the liquid within the pipe has turned tovapor and temperature is rising, which increases the liquid charging ofthe heat pipe dynamically as the temperature increases;

FIG. 5 is a graph showing thermal resistance as a function of power forthe heat sink in FIGS. 3 and 4;

FIG. 6 is a flow diagram of a method for manufacturing a heat pipe; and

FIG. 7 is a flow diagram of a method for of operation for the heat pipein FIGS. 3 and 4.

DETAILED DESCRIPTION

The disclosure and claims herein relate to a heat pipe that includes oneor more reservoirs of liquid that are closed at lower temperatures andopen at higher temperatures. The opening of the reservoirs at highertemperatures caused by higher power levels dynamically increases theamount of liquid in the heat pipe, which increases performance of theheat pipe at higher power levels. As the heat pipe cools, the liquidcondenses and flows back into the reservoirs. As the heat pipe continuesto cool, the reservoirs close. The result is a heat pipe that is moreefficient at lower power levels and still maintains high efficiency athigher power levels due to the demand-based charging of the liquid basedon temperature.

Referring to FIG. 1, a heat sink 100 is shown that includes a heat pipe105 in a U-shape with vertical portions 110 and 120 coupled to a commonhorizontal portion 130. The horizontal portion 130 is charged with afirst quantity of a liquid. The heat sink 100 includes an interface 155on the bottom surface of horizontal portion 130 that thermally couples aheat source 140 to the heat pipe 105 to transfer heat away from the heatsource 140. One example of a heat source is an integrated circuit, suchas a processor. The heat sink 100 includes multiple fins 150 as known inthe art that help dissipate heat in the heat pipe 105.

Performance of the heat sink 100 is shown graphically in FIG. 2, withthermal resistance of the heat sink plotted as a function of power forvarious levels of liquid charging in the heat pipe. Note that heat sink100 shown in FIG. 1 in the prior art would typically be charged with afixed level of liquid during manufacture then sealed, which means thatknown heat sinks have a performance defined by their fixed liquidcharge. FIG. 2 shows multiple lines that each represents performance ofa heat sink such as heat sink 100 with a different charge of liquid.Line 210 shows performance of the heat sink 100 when the heat pipe 105has a very low charge of liquid. Line 220 shows performance of the heatsink 100 when the heat pipe 105 has a low charge of liquid. Line 230shows performance of the heat sink 100 when the heat pipe 105 has anominal charge of liquid. Line 240 shows performance of the heat sink100 when the heat pipe 105 has an overcharge of liquid. And line 250shows performance of the heat sink 100 when the heat pipe 105 issaturated with liquid. At a lower power shown in FIG. 2 at 260, thethermal resistance of the heat pipe with lesser liquid charges is lessthan the thermal resistance of the heat pipe with greater liquidcharges. But the lesser liquid charges increase in thermal resistance atsignificantly lower power than for greater liquid charges. FIG. 2 showsgraphically why most manufacturers of heat sinks that use heat pipes usean overcharge of liquid or saturation of liquid in the heat pipes, sincehigher powers can lead to catastrophic failure in integrated circuits,and heat pipes with an overcharge of saturation of liquid will operateat much higher powers without a significant increase of thermalresistance.

An improved heat sink 300 is shown in FIG. 3, which includes a heat pipe305 in a U-shape with vertical portions 310 and 320 coupled to a commonsubstantially horizontal portion 330. The heat pipe 305 includes aninterface 355 on the bottom surface of the substantially horizontalportion 330 that thermally couples a heat source 340 to the heat pipe305 to transfer heat away from the heat source 340. Heat source 340could be an integrated circuit, such as a processor. In the mostpreferred implementation, the substantially horizontal portion 330overlies the interface 355 and is charged with a first quantity of aliquid. The heat sink 300 includes multiple fins 350 that are thermallycoupled to the heat pipe to help dissipate heat in the heat pipe 305.

Heat pipe 305 includes reservoirs 360 and 362 in the lower portion ofthe substantially horizontal portion 330. Reservoirs 360 and 362 arepreferably each charged with a second quantity of liquid 370 and 372.Each reservoir 360 and 362 has a corresponding temperature-actuatedvalve 380 and 382, respectively, that each has an actuation temperature.When the temperature is below the actuation temperature of thetemperature-actuated valves 380 and 382, the valves seal the liquid inthe reservoirs 360 and 362, as shown in FIG. 3, which means the secondquantities of the liquid 370 and 372 in the reservoirs 360 and 362,respectively, is isolated from the first quantity of the liquid in thesubstantially horizontal portion 330. When the temperature is above theactuation temperature of the temperature-actuated valves 380 and 382,the valves are open, as shown in FIG. 4, which means the second quantityof liquid 370 and 372 in the reservoirs 360 and 362, respectively, maynow enter the substantially horizontal portion 330, as shown by arrows410 and 412 in FIG. 4. The second quantity of liquid in each ofreservoirs 370 and 372 preferably combine with the first quantity ofliquid in the substantially horizontal portion 330, resulting in totalliquid that is equal to the sum of the first quantity and the two secondquantities. As the temperature in the heat pipe decreases, the liquidwill condense and run by the force of gravity back into the reservoirs360 and 362. In the most preferred implementation, each reservoir atleast partially underlies a vertical portion, as shown in FIG. 3 byreservoir 360 underlying a portion of vertical portion 310 and byreservoir 362 underlying a portion of vertical portion 320. This isdesirable because the condensation occurs in the upper portion of thevertical portions 310 and 320, which means the condensed liquid will rundown the sides of the vertical portions 310 and 320 into the reservoirs360 and 362. Once the temperature decreases to the actuation temperatureof the temperature-actuated valves 380 and 382, the valves 380 and 382close, as shown in FIG. 3, once again sealing the liquid 370 and 372 inthe reservoirs 360 and 362, respectively.

One suitable example of temperature-actuated valves 380 and 382 arebi-metal valves. Bi-metal valves are a composite layer made by bondingtogether two materials with different thermal expansion coefficients. Asthe one material with the greater thermal expansion coefficient expandsmore upon heating than the other, the composite layer generates atemperature-dependent deformation of the bi-metallic valve. For theexamples in FIGS. 3 and 4, we assume the exterior ends (farthest to theoutside) of valves 380 and 382 are attached to the edge of thereservoir, leaving the opposing interior portions of valves 380 and 382to deform, and thus open as shown in FIG. 4 and close as shown in FIG.3. The opposing edge, or the entire opening of the reservoir, couldinclude a valve seat the bi-metallic valve rests on when the temperatureis below the actuation temperature. The valve seat assures a tight sealwhen the valves 380 and 382 are closed so the liquid 370 and 372,respectively, is sealed in respective reservoirs 360 and 362. Bi-metalvalves are well-known in the art, and thus are not discussed in moredetail here. Various materials can be used for the bi-metal valvesdepending on the desired actuation temperature, depending on the amountof opening desired when the actuation temperature is reached, anddepending on the liquid being used. The disclosure and claims hereinexpressly extend to the use of any temperature-actuated valve, whethercurrently known or developed in the future.

By providing a heat sink with a heat pipe that includes one or morereservoirs as shown in FIGS. 3 and 4, the performance of the heat pipeincreases due to the dynamic charging of liquid into the heap pipe asthe power rises. A graph of the performance of the heat sink 300 shownin FIGS. 3 and 4 is shown in FIG. 5. This example assumes the reservoirsare filled with liquid during manufacture of the heat sink, and thesubstantially horizontal portion 330 of the heat pipe otherwise has avery low charge of liquid besides the liquid in the reservoirs. Thelinear portion 510 in FIG. 5 is the same as the linear portion of line210 in FIG. 2. We assume, however, that at a temperature just before thebend in the line 530 that shows an increase in thermal resistance for avery lightly charged heat pipe, the valves 380 and 382 open. With thevalves opened, the amount of liquid in the heat pipe increases. This iswhy the heat pipe herein has dynamic charging of liquid. The performanceat portion 520 in FIG. 5 is linear with power until a sufficient poweris reached that the thermal resistance increases dramatically, as shownat 540. Note that 540 corresponds to the upper portion of line 240 inFIG. 2. Comparing the performance in FIG. 5 to the performance in FIG. 2shows the performance of the heat sink 300 in FIGS. 3 and 4 is optimizedacross all operating temperatures. At low powers, the heat sink 300 hasthe performance of a heat pipe that is very lightly charged. But as thepower increases, the charging liquid in the reservoirs is released. Thedynamic charging of liquid in the heat pipe 305 in FIGS. 3 and 4provides much better performance than a heat pipe 105 that does not havereservoirs or valves that provide dynamic charging, as shown in FIG. 1.

FIG. 6 shows a method 600 for manufacturing the heat sink 300 shown inFIGS. 3 and 4. Note that many other steps could be included in themanufacturing process, as known in the art. One or more liquidreservoirs are provided in a heat pipe (step 610). One or more bi-metalvalves are then designed to open at a specified temperature (step 620).The reservoir(s) are filled with liquid (step 630). The bi-metal valvesare installed to cover the liquid reservoirs (step 640). The heat pipeis then charged with liquid (step 650) and sealed (step 660). It isknown in the art to evacuate all the air in the heat pipe beforesealing. Note the manufacturing process shown in FIG. 6 includes one ormore steps that are not performed when manufacturing known heat pipes.The result is a heat pipe with increased thermal performance due to thedynamic charging of liquid in the heat pipe as the temperature of theheat pipe rises.

FIG. 7 shows a method 700 that represents how the heat pipe 305 in FIGS.3 and 4 works. We assume the heat pipe is in the condition shown in FIG.3 when method 700 initially begins. As long as the temperature of thebi-metal valves is not reached (step 720=NO), method 700 loops back andcontinues until the opening temperature of the bi-metal valves isreached (step 720=YES). The bi-metal valves open (step 730), as shown inFIG. 4. The liquid in the reservoir(s) flows through the openings in thebi-metal valve(s) into the heat pipe, as shown by arrows 410 and 412 inFIG. 4. As the temperature drops, liquid condenses and flows by theforce of gravity through the openings in the bi-metal valve(s) into thereservoir(s). As long as the closing temperature of the bi-metal valvesis not reached (step 760=NO), method 700 loops back to step 760 untilthe closing temperature of the bi-metal valves is reached (step760=YES). The bi-metal valve(s) close (step 770). Method 700 is thendone.

With multiple reservoirs as shown in FIGS. 3 and 4, it is possible tohave a first valve open at a first predetermined temperature and asecond valve open at a second predetermined temperature different thanthe first predetermined temperature. The disclosure and claims hereinexpressly extend to any suitable number of reservoirs with any suitablenumber of valves that can open at the same temperature or that can openat different temperatures. For example, a heat pipe could include fourdifferent reservoirs with four different valves that each has differentactuation temperatures.

Any suitable liquid may be used to charge the heat pipe disclosedherein. The suitability of the liquid depends on factors such as thematerial used to form the heat pipe and the desired performance of theheat pipe. For heat sinks used for integrated circuits, the preferredmaterial for the heat pipe is copper, and the preferred liquid is water.

A heat pipe includes one or more reservoirs of liquid that are closed atlower temperatures and open at higher temperatures. The opening of thereservoirs at higher temperatures caused by higher power levelsdynamically increases the amount of liquid in the heat pipe, whichincreases performance of the heat pipe at higher power levels. As theheat pipe cools, the liquid condenses and flows back into thereservoirs. As the heat pipe continues to cool, the reservoirs close.The result is a heat pipe that is more efficient at lower power levelsand still maintains high efficiency at higher power levels due to thedemand-based charging of the liquid based on temperature.

One skilled in the art will appreciate that many variations are possiblewithin the scope of the claims. Thus, while the disclosure isparticularly shown and described above, it will be understood by thoseskilled in the art that these and other changes in form and details maybe made therein without departing from the spirit and scope of theclaims.

The invention claimed is:
 1. A heat pipe comprising: a substantiallyhorizontal portion charged with a first quantity of liquid andcomprising a substantially horizontal bottom surface that includes asubstantially horizontal interface for attaching a heat source; areservoir extending downwardly from the bottom surface of thesubstantially horizontal portion that contains a second quantity of theliquid; and a temperature-actuated valve overlying the reservoir,wherein the temperature-actuated valve seals the second quantity of theliquid in the reservoir when the temperature is below an actuationtemperature and unseals the second quantity of liquid in the reservoirwhen the temperature is above the actuation temperature, wherein thetemperature-actuated valve comprises a bi-metal valve that has a firstend fixedly attached to an edge of the reservoir with an opposing secondend free to move from a first position that seals the reservoir to asecond position that unseals the reservoir.
 2. The heat pipe of claim 1wherein, when the temperature is above the actuation temperature, thefirst and second quantities of the liquid combine to form a thirdquantity of the liquid that comprises a sum of the first and secondquantities of the liquid.
 3. The heat pipe of claim 1 wherein thereservoir comprises a valve seat the temperature-actuated valve rests onwhen the temperature is below the actuation temperature.
 4. The heatpipe of claim 1 further comprising at least one vertical memberconnected with the substantially horizontal portion.
 5. The heat pipe ofclaim 1 further comprising a plurality of fins that dissipate heat. 6.The heat pipe of claim 1 further comprising: a second reservoir thatcontains liquid in the lower portion of the substantially horizontalportion; and a second temperature-actuated valve overlying the secondreservoir, wherein the second temperature-actuated valve seals theliquid in the second reservoir when the temperature is below a secondactuation temperature and unseals the liquid in the second reservoirwhen the temperature is above the second actuation temperature.
 7. Theheat pipe of claim 6 wherein the actuation temperature and the secondactuation temperature are the same.
 8. The heat pipe of claim 6 whereinthe actuation temperature and the second actuation temperature aredifferent.
 9. The heat pipe of claim 1 wherein the heat pipe is made ofcopper.
 10. The heat pipe of claim 9 wherein the liquid comprises water.11. The heat sink of claim 1 further comprising two vertical membersconnected with the substantially horizontal portion to form a U-shape,wherein the reservoir at least partially underlies one of the twovertical members.
 12. The heat sink of claim 1 further comprising asecond reservoir extending downwardly from the bottom surface of thesubstantially horizontal portion that contains a third quantity of theliquid, wherein the reservoir and the second reservoir are positioned onopposite sides of the substantially horizontal interface and adjacent tothe substantially horizontal interface.
 13. A heat sink comprising: aplurality of thermally-conductive fins; and a heat pipe thermallycoupled to the plurality of thermally-conductive fins, the heat pipecomprising: a substantially horizontal portion that contains a firstquantity of a liquid and comprises a substantially horizontal bottomsurface that includes a substantially horizontal interface for attachinga heat source; two vertical members connected with the substantiallyhorizontal portion to form a U-shape; first and second reservoirsextending downwardly from the bottom surface of the substantiallyhorizontal portion, wherein each of the first and second reservoirscontains liquid and at least partially underlies one of the two verticalmembers; a first bi-metal valve overlying the first reservoir, the firstbi-metal valve sealing the liquid in the first reservoir when thetemperature is below a first actuation temperature and unsealing theliquid in the first reservoir when the temperature is above the firstactuation temperature; and a second bi-metal valve overlying the secondreservoir, the second bi-metal valve sealing the liquid in the secondreservoir when the temperature is below a second actuation temperatureand unsealing the liquid in the second reservoir when the temperature isabove the second actuation temperature.
 14. The heat sink of claim 13wherein the plurality of thermally-conductive fins dissipate heat. 15.The heat sink of claim 13 wherein the first actuation temperature andthe second actuation temperature are the same.
 16. The heat sink ofclaim 13 wherein the heat pipe is made of copper.
 17. The heat sink ofclaim 16 wherein the liquid comprises water.